Paperboard feeders are well-known in the prior art and they include various types of feeder elements which drive the lowermost sheet of a stack past a gate to the nip rolls of a box-finishing machine. One type of feeder is a xe2x80x9ckicker barxe2x80x9d which engages the trailing edge of the sheet and pushes it to the nip rolls. More recent feeders include segmented wheels which are shown in U.S. Pat. No. 4,045,015 and engage the underside of the sheet; whole wheels shown in U.S. Pat. No. 4,614,335 and U.S. patent application Ser. No. 06/674,294, filed Nov. 23, 1984, entitled xe2x80x9cRotary-Type Feeder Machines and Methodsxe2x80x9d and which also engage the underside of the sheet; and belts shown in U.S. Pat. No. 4,494,745. In these more recent feeders, a vacuum or suction is utilized to hold the sheet on the feed elements and some feeders also use a grate moveable above and below the feed elements to establish or terminate driving engagement between the sheet and feed elements.
With all of these types of feeders of the prior art, once the sheet enters the nip rolls, the feed element is disengaged from the sheet leaving the nip rolls to continue the feeding of the sheet to the next station in the box-finishing machine. It is most important that the sheet be fed to the nip rolls in xe2x80x9cregisterxe2x80x9d and with xe2x80x9cmatched velocityxe2x80x9d, meaning that the velocity of the sheet must equal the surface velocity of the nip rolls, and further that the nip rolls feed the sheet in synchronism with the moving parts of the box-finishing machine.
One of the problems attendant feeders of the prior art is that the weight of the sheet stack and the added pressure on the sheet produced by the vacuum, produces a drag on the sheet being fed resulting in loss of registry or control of the sheet. To compensate for the drag on the sheet, it is necessary to increase pressure on the sheet from the nip rolls by adjusting the spacing between the nip rolls. However this can result in crushing the paperboard sheet which, in turn, will weaken the sheet. It can also deform the surface of the nip rolls which may produce a velocity change, making it impossible to match the velocity of the sheet with that of the nip rolls, and the velocity of the nip rolls with that of the other parts of the box-finishing machine. Moreover, when feeding corrugated board having creases perpendicular to the direction of flow, control of the sheet may be lost when the crease enters the nip rolls due to the surface depression of the crease. In addition, increasing the pressure of the nip rolls accelerates the wear on the nip rolls as well as their bearings and gears, thus shortening the life of these parts and requiring repair and production downtime.
An object of the present invention is to provide novel and improved methods and apparatus for feeding paperboard blanks or similar sheets. Included herein are such methods and apparatus that may be utilized to feed paperboard blanks to a box-finishing machine in highly accurate register or synchronism with the machine and which substantially reduces, if not eliminates, the problems described above heretofore attendant conventional feeders now in use.
A further object of the present invention is to provide a novel and improved feeder capable of feeding paperboard blanks or sheets through nip rolls of a box-finishing machine in registry with the velocity of the nip rolls. Included herein is such a feeder which will positively drive a substantial length of the sheet through and in registry with the nip rolls. Another object is to provide such a feeder which may utilize feed wheels or belts which engage the underside of the blanks or sheets to drive them to and through the nip rolls.
A further object of the present invention is to provide a sheet feeder which may be adjusted as desired in accordance with the length of the blank or sheet to change the feed stroke, i.e., the distance through which the sheet is positively fed or driven to and through the nip rolls of an associated machine.
A further object of the present invention is to provide a sheet feeder having an improved drive transmission for controlling the velocity of the feeder elements. Included herein is the provision of a drive transmission that drives the feeder elements such that when the feeder elements initially engage the sheet, they will be at nearly zero or absolute zero velocity and subsequently they will be at a constant predetermined velocity for driving the sheet at said constant velocity.
Another object of the present invention is to provide in a sheet feeder, a drive transmission combining a constant velocity input and a variable velocity input to drive feeder elements from a single output. Included herein is such a drive transmission whose output varies in velocity from absolute zero or nearly zero velocity for initially engaging a sheet to constant velocity for driving the sheet at said constant velocity.
Another object of the present invention is to provide a novel sheet feeder for box-finishing machines which feeder is capable of feeding a greater number of sheets per cycle of the box-finishing machine to increase the production of the machine but without increasing the inertia load on the machine. Included herein is such a sheet feeder that may be adjusted to feed either a single sheet or a plurality of sheets per cycle of the associated box-finishing machine. Further included herein is such a feeder that will achieve the foregoing objects in a lead-edge feeder, that is, a feeder that initially engages the leading edge of the sheet to be fed.
The present invention is preferably applied in a feeder for successively driving paperboard sheets through nip rolls of a box-finishing machine in synchronism with the latter. In the preferred form of the invention, the sheets are successively fed from a lowermost position in a stack of sheets which stack is lowered on feeder elements for driving the lowermost sheet to the nip rolls. After the sheet has been fed, the sheet stack is raised to disengage the fed sheet from the feeder elements and then the stack is lowered again to engage the next sheet to be fed on the feeder elements.
In accordance with the present invention, the sheets are positively driven to and through the nip rolls at a velocity which is matched to the surface velocity of the nip rolls. In the preferred embodiment, when the sheet initially engages the feeder elements, the latter are at nearly zero velocity. Subsequently, the feeder elements are driven at a constant velocity equal to the surface velocity of the nip rolls so that the sheet is driven to and through the nip rolls at the same matched velocity. A novel drive transmission is provided allowing the sheet to be positively driven through the nip rolls along a substantial portion of the length of the sheet, and at the conclusion of the feeding portion of the drive cycle, the velocity of the feeder elements is decreased to nearly zero velocity for engaging the next sheet to be fed while at this reduced velocity. The feeding portion of the cycle is then resumed to feed the next sheet at matched, constant velocity to and through the nip rolls.
In its preferred form, the drive transmission includes a constant velocity input drive and a variable velocity input drive which are resolved at a single output for driving the feeder elements through the aforementioned cycle. The period of engagement of the feeder elements with the sheets may be adjusted to change the length of the feeding stroke to suit the particular length of the sheets being fed.